In recent years, to improve luminous efficiency in a light emitting device such as organic EL, fluorescence and LED, improvements have been studied in light emitting materials, low voltage, light extraction efficiency and the like.
As a method of improving light extraction efficiency, it is studied to introduce a light scattering layer and low refractive index layer in a light emitting device, and the like (Patent Documents 1 and 2). Further, such a method is known that the travel direction of light in the waveguide mode is polarized to the direction of the light emitting device surface using diffraction of light to improve light extraction efficiency (Patent Document 3). Furthermore, such a technique is invented that metal periodical grid structure or fine-particle dispersing element is provided near a light emitting portion in a light emitting device to excite surface plasmons, thereby extracting light to the outside with high efficiency (Patent Documents 4 and 5, Non-patent Documents 1 to 5).
In propagating surface plasmons on a metal surface, a polarized wave of free electrons generated by an electromagnetic wave (visible light, etc.) incident upon a conductive material surface such as a metal forms an electric field of the transverse wave on the surface. In the case of propagating surface plasmons existing on a flat metal surface, the dispersion straight line of propagated light does not cross a dispersion curve of plasmons, and therefore, the propagated light is not able to directly excite plasmons. However, when the periodical grid structure exists on the metal surface, diffracted light Bragg-reflected by the grid crosses a dispersion curve of plasmons, and the incident electromagnetic wave and the polarized wave of free electrons on the metal surface are capable of generating a resonance state (Non-patent Document 6).
At this point, the wave number vector of surface plasmons is a value equal to the wavelength of light, and exciton and light is coupled coherently to be exciton polariton. Polariton is a state in which the polarized wave of free electron and electromagnetic wave exchange energy by resonance. When the pitch and height of the periodical grid structure are substantially constant, i.e. when crystalline of the periodical grid structure is high, the surface plasmons have one wave number vector, are coupled to light with particular incident angle and wavenumber (wavelength), and an emission enhancement phenomenon is observed by the so-called surface plasmon resonance.
For example, in Non-patent Document 5, it is disclose that, to induce coupling of light and surface plasmons in a light emitting layer inside an organic light emitting device, a grid structure of a periodical waveform is used, thereby prevent transmission and waveguide in the transverse direction of emitted light while enhancing light output and efficiency of the structure body. According to this method, theoretically, it is possible to couple light emitted from the organic light emitting material inside the organic EL light emitting device up to maximum 93%.
Further, Non-patent Document 3 shows that luminous efficiency of blue light by UV excitation of semiconductor quantum well structure is enhanced by periodical structure of silver. Furthermore, Non-patent Document 4 shows that luminous efficiency of an LED is enhanced also by periodical structure of silver.
Moreover, the light emitting device of organic EL or the like adopts a configuration that a high refractive index region is sandwiched between low refractive index regions. Since a light emitting portion is included in the high refractive index region, light emitted in the light emitting portion becomes the waveguide mode, and is confined within the high refractive index region, while being absorbed in the waveguide process to attenuate. Accordingly, it is not possible to extract light outside the device, and light extraction efficiency significantly decreases.
In order to effectively improve light extraction efficiency, it is necessary to disturb the waveguide mode at an early stage before the emitted light attenuates. Therefore, to improve light extraction efficiency, proposed is a method for forming an intermediate refractive index layer between the low refractive index region and the high refractive index region (Patent Document 6).
However, in the method as disclosed in Patent Document 6, it is not possible to disturb total reflection, the totally reflected light maintains the waveguide mode and attenuates, and therefore, significant improvements in light extraction efficiency are not expected. On the other hand, as an example of disturbing the waveguide mode and improving light extraction efficiency, proposed is a method for extracting light confined by the waveguide mode with a periodical structure provided near a substrate as diffracted light (Patent Document 7).